The present invention relates to electrical devices and, more particularly, to power converters. A major objective of the present invention is to provide a high-precision spread-spectrum power converter. Herein, related art labeled “prior art” is admitted prior art; related art not labeled “prior art” is not admitted prior art.
There are many applications for power converters that require high precision control of a fixed output voltage. These are often traditional power supply applications, well know to those familiar with prior art. However, recent technology requirements have evolved requiring high precision control of variable output power converters, and more recently digitally controlled variable output power converters. When a digital control requirement is combined with a requirement for a variable output power controller, many limitations of prior art become evident. The present invention addresses these limitations with a novel control method.
One application for high precision, digitally controlled variable output power converters is in the field of lighting control. Precise control of voltages supplied to luminaires to achieve repeatable light output levels is a requirement in most architectural and theatrical lighting installations. Additionally, since in such applications the lighting levels are often computer controlled, these power converters need to be digitally controlled. However, the application of a digital control system to such an application has significant limitations. Key among these is the implicit nature of a digital control system to step from one value to the next. Lighting control applications require smooth transitions from one light level to the next and hence complex and costly schemes have been developed to eliminate these “steps” and achieve smooth transitions between light levels, and to prevent damage to some light sources.
Prior art power conversion for lighting applications with digital control has gone through several generations of technology. Initial prior art focus was on delivering reduced portions of the input AC line voltage waveform to the load, this being accomplished through the use of moderate speed switches which could connect and disconnect the line to load at 100 or 120 times a second (each half of a 50 or 60 Hz line cycle). This technique is referred to as phase control. While simple and robust, this solution created destructive input power line harmonics, which are presently being prohibited through regulations. In addition, such “chopping” of the input, and hence output, waveforms create sympathetic vibrations (“lamp sing”) in incandescent lamp filaments at frequencies considered annoying to human hearing.
More recent prior art addresses phase control limitations by alternately coupling and decoupling an input voltage to an output many times per voltage half-cycle, rather than just once. The resulting chopped waveform can be re-integrated to provide a smooth output waveform of voltage reduced as a function of a chopper switch duty cycle. To provide for dimming and precise voltage adjustments, a pulse-width modulator can provide pulse trains with variable duty cycles to control the chopper switch. While analog pulse-width modulators are known, digital pulse-width modulators provide precise control over duty cycles more economically.
One problem with digital pulse-width modulators is that duration values change in discrete steps so that durations between steps are not available. For example, consider a counter driven by a one-megahertz (1 MHz) clock signal. Fifty counts yields a 50-microsecond duration, and fifty-one counts yields a 51-microsecond duration. A 50.5 microsecond duration is not available.
One approach to achieving an intermediate duration is to increase the clock speed. For example, if a 2 MHz clock is used, counting to 101 provides a 50.5 microsecond duration. However, other intermediate values, such as 50.25 are not available without doubling the clock frequency again. Depending on the application and technology, increasing clock speeds becomes cost-prohibitive because many circuit components must be upgraded to minimize parasitic capacitances and inductances to handle higher frequencies. Also, the bit length of counters may have to be increased to maintain the same range for available durations. The economical limit to the clock frequency and counter width is generally dictated by the specifications of cost-effective commercially available microcontroller designs.
If a higher clock frequency is not available, intermediate values can still be achieved on a time-averaged basis. For example, alternating between 50 and 51 counts can provide a time-averaged duration of 50.5 counts. For another example, a pattern such as 50, 50, 51, can achieve a time-averaged duration of 50.33 counts. In principle any intermediate value can be approached with any precision over enough cycles on a time-averaged basis. This approach, in which two consecutive counts (e.g., 50 and 51) are alternated to achieve an intermediate value on a time-averaged basis, is called “dithering” herein.
Another problem with digital power converters is that they generate electrical noise at the switching frequency. For example, cycling a chopper switch at 50 microsecond cycles yields noise in the 20 kHz region and at many higher harmonic frequencies. As this noise may interfere with the operation of other devices and exceed levels permitted by government regulations, it may need to be filtered out. A filter designed to remove this noise to comply with governmental regulations, when all the noise energy is concentrated at a single frequency, can add considerable expense to a power converter.
This electrical noise problem can be made more manageable by varying the switch cycle duration from cycle to cycle. This spreads the noise spectrum so that the peak energy delivering the electrical noise at any one frequency is lower. This makes filtering it less expensive
The problems of limited precision in output voltage and an unfavorable noise signature can be addressed separately as described above. What is needed is an approach to power conversion that can achieve high precision output control while also achieving a favorable noise signature of a spread-spectrum power converter.